Sampling and testing materials among a manufacture progress, and process simulation with dummy materials to determine machine failure or work station failure are the most common methods in quality control during a manufacturing process for products like semiconductors or thin film transistor liquid crystal display (TFT-LCD).
The sampling analysis is based on the assumptions that the manufacturing process is under static conditions, and the testing result of small samples can be representative of the real manufacturing environment as a whole.
However, the testing result of the small samples may not represent the real manufacturing environment due to imprecise sampling programs or an inappropriate tolerance acceptance levels. In this case, some potential crisis can be omitted and the broken or malfunctioning machines (hereinafter referred to as “excursive machines”)/stations cannot be found until a lot of invalid products are produced, or until the yield is reduced sharply.
Alternatively, the estimation of simulation parameters may depend on an engineer's subjective judgment that may result in some subjective faults and the suspect excursive machines will not be identified in time. Furthermore, the dummy material testing may cause additional cost.